This invention relates to the art of thermal printing, more particularly to an improved method and circuit for performing historical control of printing darkness.
Because of their simple construction, quiet operation, and low cost, thermal printers are widely used as output devices of facsimile machines, word processors, and personal computers. A thermal printer has a number of resistive heating elements equal, for example, to the number of dots across a page in the horizontal direction, or the number of dots per character in the vertical direction. Printing takes place as a page is scrolled past these resistive heating elements, or as the resistive heating elements travel across the page on a movable print head. When a dot is to be printed, current is fed to the corresponding resistive heating element, raising its temperature. The heat thus generated darkens the adjacent portion of the page, if thermosensitive paper is used, or causes transfer of ink from a ribbon to the page in printers using plain paper.
A problem in thermal printing is that when a resistive heating element prints a series off dots, the dots tend to get successively darker because of residual heat. This problem can be overcome by regulating the amount of printing current fed to the resistive heating element according to the printing or non-printing of the last few dots, a technique known as historical control.
A prior-art method of historical control supplies a current pulse of a certain length to print a dot, but masks parts of that pulse for dots printed by the same resistive heating element in the previous two lines. (The "line" may be horizontal or vertical, depending on the configuration of thee printer.) This method improves the appearance of the printed output, but does not achieve completely uniform dots; more refined forms of historical control are necessary. Attempts at refining historical control, however, lave encountered certain problems.
One problem is that the heating curve of a resistive heating clement is not linear, so simplistic schemes of regulating the amount of current according to the number of historical dots printed are unsatisfactory. An ideal solution would be to track the printing history of the resistive heating clement through a large number of past dots and calculate the optimum current to be fed for every historical pattern of dots, but this scheme would encounter a second problem: the necessary control circuitry would be highly complex and require an impractically large number of signal lines.